The subject of the present invention is an expansible acetabular prosthesis with double mobility.
The technical field of the invention is the production of surgical equipment implantable in the human body.
The main application of the invention is the production of a hip prosthesis for replacing the cotyloid cavity of the hip bone, the whole prosthesis being intended to be anchored without cement.
Total hip prostheses comprise two elements:
1. The femoral stem: this component replaces the head and neck of the femur. A stem, forming a continuation of the neck, ensures anchoring thereof in the femoral bone, with or without cement.
2. The acetabular component: this replaces the osseous acetabulum and articulates with the prosthetic femoral head. A hemispherical cavity forms the sliding surface. This element can be made in one piece from metal, polyethylene or ceramic. However, to allow it to be fixed without cement, many models are made up of the following two elements:
a) a metal cup fixed in the bone,
b) a plastic insert, in particular of polyethylene, fixed in the cup and articulating with the prosthetic head.
The problems encountered in surgical practice are the wear between the femoral head and the acetabular component, and also the fixation of the metal cup in the bone and the wear observed between the insert and the metal cup.
A number of ways of fixing the metal cup are presently employed, in particular sealing (a), impaction by force (b), screwing (c), or fixation by expansion (d), all of which are discussed below.
a) Sealing involves interposing an acrylic-type cement between the bone and the cup, which anchors in the bone and ensures the fixation of the component. However, this technique has disadvantages associated with the characteristics of the cement and of the bone and can lead in the medium term or long term to loosening.
b) Impaction by force involves press-fitting the metal cup into a bone cavity prepared with the aid of hemispherical reamers which calibrate the osseous acetabular cup to a diameter which is slightly smaller than that of the metal cup.
A coating or roughening of the outer surface of this cup improves the immediate mechanical stability of the cup and permits secondary stabilization by regrowth of bone. However, this initial mechanical stability is uncertain because it largely depends on the way in which the bone is prepared and especially on its hardness.
Most of the models therefore propose a complementary fixation recommended in cases where the press-fit retention is deemed insufficient. This fixation is ensured by screws which are placed in the bone via holes formed in the metal cup.
This technique of impaction by force has the following disadvantages:
the possible micro-movements between the screw and the metal cup can release particularly damaging metal products of wear, and
the possible contact between the screw head and the insert is a possible source of wear and creep. Another problem is the creep of the insert (if it is made of polyethylene) relative to the screw holes, since this produces debris from wear which can migrate through the screw holes and lead to progressive osteolysis, which promotes mobilization of the implant.
c) Screwing of the acetabular cup involves using a metal cup with a threading on its outer surface, which threading is screwed into the bone. This method of fixing is little used at present because the position of the component is uncertain at the time of fitting. This is because the press-fit or screw-in acetabular cups cannot be fitted with precision because their positioning can be deviated by the bone during fitting.
d) In fixation by expansion, a metal cup is used which is designed to be opened by an intermediate element of metal or polyethylene situated between the cup and the insert, for example a ring.
Different types of acetabular prostheses fixed by expansion are known, and these comprise three elements, which are:
an artificial acetabular cup which is a cup provided with expansion slots which are symmetrically and uniformly distributed at angles over its periphery and which define segments or portions of a spherical cap sector;
a threaded intermediate element which is:
a) a ring intended to be screwed into the acetabular cup, as is described in patent application EP 486 403, or
b) an intermediate core which comprises a threaded finger forming an appendix on its pole and which cooperates with a tapped hole in the artificial acetabular cup, as is described in patent application FR 2 700 946;
a friction insert which bears or engages on said intermediate element having a sliding surface in the shape of a hemispherical cavity, and in which an artificial head ball engages to restore the articulation of the hip.
The present invention concerns expansible acetabular prostheses with three elements, as are described above. More particularly, the present invention concerns acetabular prostheses with three elements substantially of spherical cap shape.
With the expansible acetabular prostheses which are presently available, the plastic insert which has to take up all the loading on the hip may creep and thus deform inside the acetabular cup.
In the case of an intermediate element with a continuous surface which better supports the insert, the acetabular cup is fixed by screws and this again does not ensure good overall stability: the reason being that the components of the prosthesis retain a certain mobility and elasticity which are also incompatible with good stability over time.
The object of the present invention is to remedy these disadvantages in particular, and the present invention aims to make available a prosthesis having:
improved mechanical stability,
easier and more precise fitting in the bone,
reduced wear of the components,
the possibility of permitting double mobility by virtue of an insert which is movable in the intermediate element.
To do this, the subject of the present invention is an acetabular prosthesis comprising:
a hollow artificial acetabular cup, preferably with an outer surface of the spherical cap type, in particular hemispherical, having expansion slots distributed over its periphery;
an intermediate cup-shaped element, with an inner surface in the shape of a spherical cap, in particular hemispherical;
a hollow insert with outer and inner surfaces of spherical cap shape, preferably hemispherical, fitting in the intermediate element;
said acetabular cup and said intermediate element comprise screwing means so that the outer surface of said intermediate element can come into congruent contact, by screwing, against at least one internal meridian generatrix of each segment delimited by two of said consecutive slots,
said means of screwing said intermediate element into said acetabular cup comprise a peripheral threading formed on the inner surface of the acetabular cup near its opening, in which threading said intermediate element is screwed with the aid of a complementary threading on its outer surface near its opening,
the internal diameter of the opening of said acetabular cup, in the standby position before screwing of said intermediate element, is smaller than the external diameter of said intermediate element, and
said acetabular cup is equipped with bone-anchoring teeth on its outer face.
This configuration of the acetabular prosthesis according to the invention in which the intermediate element bears in contact on at least one internal meridian generatrix of the internal cavity of the artificial acetabular cup permits a good distribution of the forces and stresses across the entire surface of the artificial acetabular cup.
In addition, the peripheral threading according to the present invention ensures progressive opening of the slotted element, so that the positioning of the acetabular cup at the time of fitting is made more precise.
More particularly, the intermediate element is a hollow core with a hemispherical outer surface and is able to come into contact against at least one internal meridian generatrix of each segment defined by two consecutive expansion slots of the artificial acetabular cup. This intermediate core is preferably a solid component whose outer surface is continuous. All the internal surfaces of the acetabular cup are advantageously in contact with the outer surface of the core when the latter is screwed fully and positioned in the acetabular cup.
To permit this screwing, the inner part near the opening of the hollow acetabular cup comprises a threading in which said intermediate core is screwed, said intermediate core also having a corresponding threading in the area of its upper outer edge and surrounding its own opening, and the internal diameter of the opening of said acetabular cup in the standby position before screwing of said intermediate core is smaller than the external diameter of this intermediate core.
The result is a novel expansible acetabular prosthesis with double mobility which affords solutions to the problems which are posed by the disadvantages of the present prostheses set out above: this is because the three elements which make up the prosthesis according to the invention are all of congruent and engageable hemispherical shape. Thus, because of the continuous contact of the outer surface of the intermediate component with the inner surface of the lower acetabular cup, and which are perfectly congruent once in place, a better bearing is obtained than in the case when the intermediate component is only a ring and/or is fixed to the acetabular cup only by way of threads permitting their respective screwing. The present invention avoids any mobility and displacement between these components over the course of time.
Moreover, the continuous inner surface of the intermediate element similarly permits a perfect congruence of the plastic insert, which will therefore not be at risk of creeping or catching on any roughening of the intermediate element.
The fact that the three components of the prosthesis according to the present invention are in contact and congruent with one another ensures contact without elasticity: this concept is very different from most of the patents published on this type of prosthesis and even runs counter to the teaching published to date, for example that of patent EP 640325, since these documents, by contrast, generally strive for a minimum of elasticity between the different components.
By virtue of this engagement and the congruence of the intermediate element with the acetabular cup, the present invention makes it possible to regulate the orientation of the insert which then receives the head ball of the joint, as is described in the preceding patent application by M. Chauvin, cited above.
Moreover, the particular position and arrangement of the outer anchoring and pre-impaction teeth of the acetabular cup according to the present invention make it possible, on the one hand, to place the latter in the bone cavity easily and precisely by simple bearing on the bottom of the latter without having to form a thread in the wall of this cavity, thereby permitting stabilization of the acetabular cup in the bone so that screwing of the intermediate elements is possible, and, on the other hand, to anchor in this bone cavity by expansion during screwing of the intermediate core, the orientation of the fins which form said anchoring teeth ensuring both anti-rotation and anti-extraction of said acetabular cup. Very effective initial stability is obtained by virtue of the join thus obtained between the prosthesis and the bone.
In a particularly advantageous embodiment, the anchoring teeth are distributed uniformly over the outer surface of said segments of the artificial acetabular cup toward its opening, and, before screwing, the distal ends of said anchoring teeth are situated inside the enveloping surface, preferably an enveloping sphere of diameter D5 corresponding to the position of the outer surface of the acetabular cup once expanded by screwing of the intermediate element.
More particularly, the anchoring teeth arranged on the outer surface of the segments toward the opening of the acetabular cup are formed as fins, some being symmetrical with respect to a meridian plane passing through the polar axis of the acetabular cup, and the others being symmetrical with respect to a plane perpendicular to said axis.
According to an advantageous alternative embodiment of the invention, said segments preferably comprise impaction teeth consisting of spikes situated in the polar zone of the outer surface of the acetabular cup.
The zone near the opening of the acetabular cup, in which the anchoring teeth are arranged, and the polar zone, in which the impaction teeth are arranged, are delimited by a plane perpendicular to the polar axis in a proportion of 2/3 to 2/5 for the zone near the opening and of 1/5 to 1/3 for the polar zone.
Said impaction spikes are arranged in a vertical direction or an intermediate direction between the vertical direction of the polar axis and the radial direction of said sphere of diameter and
the bone-anchoring teeth are arranged in a radial direction or an intermediate direction between said radial direction and the direction perpendicular to the polar axis.
In an advantageous embodiment, the inner surface of said intermediate element is solid and polished, and said insert consists of a cup, which is fixed or movable, preferably movable, whose outer surface is the shape of a smooth spherical cap and which has a cavity with a smooth spherical inner surface.
The possibility of using a movable cup articulating with the inner surface of the polished intermediate element permits greater stability of the prosthetic articulation upon extreme movements, which reduces the risk of luxation.
In another embodiment, it is possible to use a fixed insert: in this configuration the invention also affords the advantage of perfect congruence between the insert and the cup, so as to reduce wear and creep.
The characteristics according to which the inner surface of the intermediate element is smooth and/or this surface is solid, that is to say perfectly regular and nonperforated, with no roughness or cavity (screw hole), represent an important advantage because this avoids the production of debris from wear of the insert, particularly when it is made of polyethylene.